Conventionally in such a system, the beds are operated in pairs or triplets or groups of other numbers of beds, so that when one or more beds of the system are operating in a venting phase, at least one bed is operating in a charging phase, so that oxygen enriched gas is continuously being produced.
It is essential for the integrity of the system, to protect the molecular sieve bed material of the beds from contamination such as particulate contamination, e.g. dust, which is most likely to occur during installation in the aircraft, and from water ingestion which is most likely to occur during storage of the molecular sieve bed material. Also components and fittings for a bed tend to be particular for that bed.
Thus to prevent the likelihood of contamination, water ingestion and the exchanging of components and fittings between the beds, groups of the beds e.g. two beds which are to be operated in tandem, are conventionally provided as modules with all associated valves etc. to enable the material of the bed to be isolated during storage and installation, ready to be plumbed into supply and distribution conduits of the aircraft.
However such modules thus tend to be heavy and difficult to handle during installation and removal e.g. for repair or replacement.
It has been proposed in U.S. Pat. No. 5,549,736 to provide a modular sieve bed so that the area/volume of the sieve bed material available for non-product gas adsorption can be tailored to a desired product gas producing capacity. However, the modules of the arrangement described are assembled by connecting together fittings which provide manifolds for the inlet and outlet gases, such that although the arrangement relieves the problem of overly heavy and bulky multiple bed modules, the molecular sieve bed material is not isolated during and prior to installation of the modules, making contamination and water ingestion a possibility.